1. Field of the Invention
The present invention relates to a signal processing circuit, a vibration detection circuit, and an electronic device.
2. Description of the Related Art
Referring to FIG. 8, a conventional signal processing circuit is described by exemplifying a typically known occupant protection device using an air bag. It is noted that the occupant protection device uses the signal processing circuit in order to fire a detonator for inflating the air bag based on impact detected by an acceleration switch and an acceleration sensor.
A signal processing circuit 100 illustrated in FIG. 8 for a normally open acceleration switch used in the occupant protection device includes an acceleration switch 101, a microcomputer 102, and an acceleration sensor 103. A first resistor 104 and a second resistor 105 are connected in series between a power supply line and the ground. A connection point between the first resistor and the second resistor is grounded via the acceleration switch 101. The connection point is further connected to a first port 102a and a second port 102b of the microcomputer 102 so that the first port is set as an input port and the second port is set as a Low level output/input port.
When vibration of a predetermined value or more is applied to the acceleration switch 101, the acceleration switch becomes the closed state, and the microcomputer 102 detects the open/closed state of the acceleration switch based on a voltage change via the second port 102b. The microcomputer 102 subsequently detects an output of the acceleration sensor 103, and, when it is determined based on the output result that the vibration of the predetermined value or more has been applied to the acceleration sensor, the microcomputer 102 switches switching field-effect transistors 106 and 107 into the closed state, to thereby supply an ignition current to a detonator 109 from the power supply line via a DC/DC converter 108.
In the signal processing circuit used in the occupant protection device described above, the microcomputer needs to be operated all the time in order to constantly detect the vibration applied to the acceleration switch. The microcomputer also needs to drive the sensor constantly and prepare for detection of the output signal of the sensor. Those technologies are described in a publication of Japanese Patent Publication No. 2002-55114.
In the above-mentioned signal processing circuit using the acceleration switch, the sensor, and the microcomputer in combination, it is necessary to constantly drive the sensor for detecting vibration or acceleration and also supply power for operating the microcomputer. Particularly in the case where the signal processing circuit is incorporated in a device that can mount only a small-capacity battery, it is necessary not to waste electric power of the battery by configuring a system including the signal processing circuit so as to stand by when no vibration is detected and operate in response to the detection of vibration.